Method of reconditioning furnace tubes



July 14, 1942. E. s. DIXON ETAL H METHOD OF RECONDITIONING FURNACE TUBESFiled Dec. :29,v 1937 K2 P & Ask- +0) W INVENTORS 'IEII llll llllll.

fih H- W ATTORNEY Patented July 14, 1942 UNITED STATES PATENT OFFICEMETHOD OF RECONDITIONING FURNACE- TUBES Application December 29, 1937,Serial No. 182,174

13 Claims. (01. 196-69) Our invention relates to the cleaning of tubesand more particularly to the removal of coke or carbonaceous depositsfrom tubes employed in the conversion of hydrocarbon fluids.

When hydrocarbon fluids are subjected to elevated temperatures whileflowing through banks of tubes suitably positioned within a furnace,deposition of coke or carbonaceous material upon the inner walls of thetubes comprising the heating coil is commonly encountered. The extent towhich such deposition of coke or carbonaceous material within the tubeswill take place is dependent upon the characteristics of the hydrocarbonfluid undergoing treatment and time and temperature conditions of thetreatment, but it is nevertheless invariably encountered to a greater orlesser degree regardless of the cleanliness of the material treated. Theproblem of coke deposition is especially prominent in processes whereinthe hydrocarbon fluids treated are subjected to temperatures at whichdecomposition of constituents of the material processed takes place, asfor example, in the cracking of hydrocarbon oils. By reducing theavailable crosssectional area of the tube and greatly reducing theefliciency of heat transfer through the walls of the tube, such depositsupon the inner walls of the tube effectually impede the operation of theprocess.

Such tubes must therefore be subjected to periodic cleanings to removethe objectionable deposits. Various methods have been proposed for theremoval of carbon from the inner walls of tubes but in practicing thesemethods it has been found that substantially complete removal of thecoke-like deposits can be effected only by methods involvingsubstantially complete oxidation of the deposit, resulting intemperatures harmful to the tube Wall or by supplementing the otherpreviously disclosed methods by a removal of a considerable remainingportion of the deposit by means'of mechanical coke knockers or. by meansof localized burning. Such a supplementary operation necessitates theremoval at each cleaning of headers, return bends or the like. methodsare ineflicient, not only because of the time lost inwaiting for thetubes to cool to a temperature at which headers, return bends or thelike can be removed, but also because the frequent removal of headersreturn bends or the like materially aflects the life of the mechanicaljoints.

Our invention has for an object the provision of a method whereby cokeor carbonaceous materials deposited upon the inner walls of tubes orThese coils employed in the conversion of hydrocarbon fluids can beefficiently and substantially completely removed in a single completeoperation thus eliminating the necessity of the supplementary mechanicaloperation and the removal of headers, return bends or the like.

Our invention has for another object substantially complete removal ofcoke-like deposits from tubes by a method which completely eliminatesdamage of breakage due to mechanical shock at low temperatures.

A further object of our invention is the provision of a method for theremoval of carbonaceous deposits from fluid heating tubes wherein severetemperature conditions, harmful to the tubes, are avoided.

Another object of our invention is to provide a method for thereconditioning of tubes previously employed for the conversion ofhydrocarbon fluids, such as the cracking of hydrocarbon oils, in whichmethod a simultaneous decoking and heat treatment of the tubes iseffected which relieves any strains produced in a foregoing conversionoperation and results in a decided reduction of tube failures duringsubsequent conversion operations.

An additional object of our invention is the relieving of anyembrittlement of the tubes and fittings.

We have found that when tubes or coils positioned in a furnace andcontaining a deposit of coke or carbonaceous material, such as isdeposited during the conversion of a hydrocarbon fluid, are graduallyraised to and maintained for a suitable time at a critical temperature,while steam is passed through the tubes, there is effected a breaking ofthe tube-to-carbon bond and spalling of the coke-like deposit isinitiated. We have further found that by subsequent judicious control ofthe furnace temperatures, the judicious addition of small amounts ofair. and the control of the direction of flow of the steam and air, cokedeposits can be emciently and substantially completely removed. Theadded air is thought to promote spalling of the coke which is blown outwith the steam as a granular mass or as a powder. Actual burning of thecoke may be carried On to some extent but only to a limited extent andas a last resort.

According to our invention coke or carbonaceous material, deposited uponthe inner walls of tubes or coils during the process of convertinghydrocarbon fluids, is removed from these tubes or coils in conformitywith the method comprising the following steps:

(a) Raising the tube temperature to 1250 F.- 1450" F., preferably toabout 1350 F., while pass ing steam through the tubes positioned in thefurnace.

(b) Maintaining the tube temperature at 1250 F.-l450 F., preferably atabout 1350 while a substantial spalling of coke is taking place. Duringthis period small amounts of air may be added to the steam to promotespalling, but burning of coke to any appreciable extent within the tubesis avoided.

(c) When the greater part of the spalled coke has been blown out,gradually lowering the furnace temperature and while doing so adding airin slowly increasing proportions of air to steam until finally air aloneis being passed through the tubes. The furnace temperature is cooled tosuch a point that the tubes are at the lowest temperature at which theadded air will effect decomposition of coke in the tubes. In practice wehave found this temperature to be in the neighborhood of 1100 F.

(d) Raising the tube temperature to a point immediately below that atwhich scaling or change in the metal structure of the tube might resultand continuing the flow of air in order to eifect decomposition of anycarbonaceous material whichmay have survived the spalling operation.This temperature maybe of the order of 1250 F. to 1450" F. or higherdepending upon the chemical composition of the tube metal.

(e) As a final step, reversal of the flow of air may be made in eachdirection in order to insure the thorough scavenging of all loosematerial and to efiect the uniform distribution of the high temperaturesnecessary to burn away any remaining coke.

The method of decoking tubes in accordance with our invention will bemore fully understood from the following description read in connectionwith the accompanying drawing which shows a diagrammatic representationof an oil cracking coil, suitably positioned in a furnace, and providedwith one form of equipment suitable for coke removal according to themethod of the invention.

Referring to the drawing, coil 2 represents a single, or several banks,of tubes such as employed in the conversion of hydrocarbon fluids, forexample, the cracking of hydrocarbon oils, and which are suitablypositioned in a furnace I. It is not intended that the representationsof furnace I' and coil 2- be limited to any specific type used in aspecific process, for the method of the invention is applicable to anyof the many types of heating coils used in the conversion of hydrocarbonfluids and is not limited in any manner by the nature of the processemployed to effect the conversion of the hydrocarbon fluid.

During the process of conversion the hydrocarbon fluid to be processedis forced, by means not shown in the drawing, through line 3 controlledby valve 4 into the coil 2. The hot products of conversion leave coil 2through line 5 controlled by valve 6. When deposition. of coke orcarbonaceous material upon the inner walls of the tubes comprising coil2 becomes such as to necessitate reconditioning of the tubes or forother reasons, the unit is shut down in a conventional manner. Uponcompletion of this phase of the operation valves 4 and 6 are closed toisolate the heating coil from the rest of the unit.

Steam lines I, 8, 9 controlled by valves I 0, II, I2 and air lines I3,I4, I5 controlled by valves I6, I'I, I8 are manifolded to lines I9, and2| connected respectively to the inlet, an intermediate point of, andthe outlet of coil 2. Lines I9 and 20 are controlled by means of valves22 and 23 respectively. Lines I9, 20 and 2| establish communicationbetween coil 2 and line 26 controlled by valves 24 and 25.

To initiate the reconditioning operation of the tubes after theparticular conversion process is discontinued, the furnace temperatureis gradually raised to a temperature of from 1250 F. to 1450 F.,preferably about 1350 F. Valves 4, 6, 22 and 23 are closed, valves 24and 25 are opened, and steam is admitted through line I controlled byvalve IE! and passed through line I9 into coil 2. Steam leaves theoutlet of coil 2 and flows through lines 2I and 25 to be exhaustedthrough line 21 to a stack or other suitable place of disposal. Anelevated steam pressure, preferably above lbs. per sq. in., andpreferably of about to lbs. per sq. in. at the inlet of the coil ismaintained during this initial phase of the operation. When thesepreliminary heating conditions are carefully maintained for a period oftime, usually not exceeding about three hours, a spalling action isinitiated comprising the breaking of the tube-to-coke bond and thedisintegration of the carbonaceous deposit into a broken mass which mayvary in nature from a granular to a powdery material.

A substantial portion of the coke deposit becomees detached from coil 2during this spalling operation and is blown from the coil by the steamwhich also functions as a vehicle for the spalled coke which iteliminates from coil 2 through lines 2|, 26 and 21. It is sometimesfound necessary to promote spalling by the intermittent addition ofsmall amounts of an oxygen-containing gas, for example air which, inthis particular stage of the cleaning operation, is introduced throughline I3 controlled by valve IS. The amount of air required to promotespalling varies with the type of deposit; often none is needed and atother times addition of as much as ten per cent or more of the steampasses through coil 2 is required to initiate the desired effect. Itshould be pointed out thatthis addition of oxygen-containing gas isessentially to initiate and promote the spalling effect peculiar to thistemperature range. The manner in which this addition of theoxygen-containing gas initiates and promotes the spalling action is notdefinitely known. However, it is thought that the spalling of coke isdue in measure to the difference existin; between the value of. thecoefficient of expansion of the metal of the tube Wall and that of thecoefiicient of expansion of the carbonaceous deposit and to thedifferences in the coefficients of expansion of varying sections of thecarbonaceous deposit itself, and it is believed that upon introducing alimited quantity of an oxygen-containing gas into coil 2 at theoperating conditions, the restricted combustion of coke with itsinherent sudden temperature change effects the breaking of thetube-to-carbon bond and the simultaneous breaking-up of the carbonaceousdeposit. This mechanical disintegration of the carbonaceous deposit isthen accelerated without further combustion. of coke by the suddencooling effect of the steam flowing through the coil, in the absence ofany oxygencontaining gas, at a temperature low-er than that of the metalof the tube wall. This mechanical disintegration of the carbonaceousdeposit induced, if necessary, by the combustion of only a smallfraction of the deposit, supplemented by lated to prevent erosion of thelines.

the further loosening effect of the steam flow is believed to constitutethe major part of the spalling action by which means substantially allof the carbonaceous deposit is removed from the tube in solid form as apowdery or granular mass that has undergone neither chemical change norchange of state. Once the spalling action is well under way the steampressure should be regu- Regulation is based on Visual observation ofthe quantity and character of the coke discharged. Discharge of coke,exhausted steam and air leaving the systemthrough line 21 is effectedwithin a suitable receptacle or stack and subsequent reaction of thedischarged materials is prevented by the addition of a quenching medium,such as water, through line 29 controlled by valve 28. When the spallingaction ceases, substantially all of the coke deposit will have beenremoved from coil 2. Various reversals of the direction of flow throughthe coil of the steam fiow may be made to promote complete scavenging ofloose material. a

The phase of the cleaning operation subsequent to the spalling operationserves to remove by combustion any residual coke within coil 2 which mayhave escaped the primary phase of the operation. The transition from thespalling operation to the second stage is efiected gradually withoutinterruption of the process. When the spalling action begins to decreasein intensity, which will usually be about two hours after spalling hasbeen initiated, the temperature of the furnace is gradually reducedwhile admitting an oxygen-containing gas, for example air, inprogressively increasing quantities into coil 2 through lines I3 and I9and simultaneously decreasing the flow of steam through coil 2 bygradually closing valve Hi. When spalling has ceased completely thefurnace temperature is allowed to drop to 1000" F.-1200 F., and valveii] is closed gradually until only the oxygencontaining gas is passedthrough coil 2, the tubes being kept under careful observation.O'ccasional additions of steam may be made to the system through line Iduring this phase of the operation to prevent local overheating of thetubes. The passage of the oxygen-containing gas is continued until thereis no evidence of coke-combustion within coil 2. The furnace temperatureis then gradually raised to approximately 1250 F. while the air flow iscontinued to remove any coke that may not have been affected at thelower temperatures. When it is apparent that there is no furthercoke-combustion within coil 2 with the furnace temperature maintained atabout 1250 F. the flow of the oxygen-containing gas through coil! isreversed by closing valves i6, 24, 25 and opening valves l2 and 22. Theoxygen-containing gas admitted to the system through line 9 controlledby valve l2 will flow through line 2| into coil 2 to burn out the lastremnants of coke deposit. By thus reversing the flow, final remnants ofcoke deposits, which may have remained in coil 2, for example in thatpart of coil 2 which was the cooler extremity during the previous fiow,will be effectively removed. When all evidence of coke combustion Withinthe tubes has finally ceased, as indicated by visual observation or bychemical test of the effluent gas for carbon dioxide the furnace firesare extinguished while the flow of the oxygencontaining gas iscontinued.

The method of reconditioning tubes in accordance with the invention isof course not limited to the specific steps mentioned in theillustrative operation of the method. To obtain and maintain thecritical conditions given for the spall ing operation, and to effect thesubsequent removal of final traces of coke deposit, it may be desiredand sometimes be advantageous to depart or enlarge upon the specificsteps of the operation as described without departing from the spiritand scope of the invention. One of such possible additional steps isshown in the drawing. A line 20 controlled by valve 23 leads from anintermediate point of the coil into line 26. A steam line 8 controlledby valve II and an air line H controlled by valve H are manifolded intoline 20. The intermediate point of the coil 2, at which connection withline 20 is made, is preferably a point immediately preceding thatsection of coil 2, at which the carbonaceous deposit is heaviest. Byclosing valve 23, and suitable manipulation of valves I I and I1, air orsteam or both may be introduced into coil 2 to assist in any phase ofthe operations mentioned in the illustrative example. The method mayfurther be modified by interrupting any phase of the operation byclosing of valve 23, opening of valves 22, 24 and 25 and theintroduction of air or steam or both into coil 2 through line 20 toeffect the discharge of coke, ash or both from coil 2 through line-2|and line l9 simultaneously.

Other modifications, such as the interruption of any of the stepscomprised in the illustrative example of the invention for the purposeof passing only steam through a part or all of coil 2 in order to heator cool any part or all of coil 2, will, of course, be apparent to oneskilled in the art. It is not intended that such obvious modificationsbe considered beyond the scope of our invention, nor is it intended thatour method be limited by omission of any obvious modificatory steps fromthe illustrative example.

The steps of reconditioning hydrocarbon fiuid heating tubes, accordingto the invention, comprising the gradual heating of these tubes to thecritical temperature range in which spalling of the carbonaceous depositis brought about as set forth in the illustrative example, theirmaintenance at these temperatures within the prescribed limits of timeand their subsequent slow cooling, comprise a very beneficial heattreatment resulting in the release of contained stresses andembrittlement and a great reduction in the number of tube failures. Thisfeature of the reconditioning operation is an important one and isfurthered by judicious control of the flexible time element of thereconditioning process. We do not intend to limit ourselves to anyspecific temperature ranges or steam and air pressures and we prefer towork at the highest temperatures to which it is practicable to heat thetubes.

In practicing the method of reconditioning heating tubes in accordancewith the invention man modifications and variations may be made withoutdeparting from the spirit of the invention and therefore only suchlimitations should be imposed as are indicated in the appended claims.

We claim:

1. The method of removing carbonaceous material from tubes positioned ina heater previously employed for heating a hydrocarbon fluid to atemperature at which carbonaceous material is deposited upon the innerwalls of said tubes which comprises the steps of subjecting the tubes bymeans of external heating to an elevated temperature, continuouslypassing steam throughsaid tubes while at an elevated temperature toeffect spa'lling of the carbonaceous deposit within said tubes andeffect removal of carbonaceous material as a granular or powdered mass,periodically adding controlled quantities of gas containing free oxygento said steam to promote further spalling of said carbonaceous deposit,maintaining the temperature conditions and the fiow of steam throughsaid tubes until the spalling action substantially ceases, reducing thetemperature to which said tubes are subjected, continuously passing gascontaining free oxygen through said tubes while at a, lower temperature,gradually raising the temperature to which the tubes are exposed whilecontinuing said flow of gas containing free oxygen, thereafter reversingsaid flow of gas containing free oxygen and graduall reducing thetemperature to which the tubes are exposed.

2. The method of removing carbonaceous material from tubes which havebeen previously employed to heat a hydrocarbon fluid to a temperature atwhich carbonaceous material is deposited upon the inner walls of thetubes which comprises externally heating said tubes to an elevatedtemperature, continuously-passing steam through said tubes to effectspa-lling of the carbonaceous material within said tubes and efiectremoval of carbonaceous material as a granular or powdered mass,intermittently adding controlled quantities of gas containing freeoxygen to said steam to further promote spalling of said carbonaceousmaterial, maintaining said temperature conditions and steam flow untilthe spelling action substantially ceases, thereafter slowly increasingthe proportion of gas containing free oxygen to steam while graduallylowering the temperature of said tubes to the lowest temperature atwhich said added gas containing free oxygen will efiect decomposition ofsaid carbonaceaus material within said tubes until finally gascontaining free oxygen alone is passed through said tubes, raising thetemperature of said tubes to a temperature below that tem perature atwhich scaling or change in the metal structure of said tubes results andmaintaining the flow of said gas containing free oxygen to substantiallycompletely remove any remaining carbonaceous material.

3. The method of removing carbonaceous material from tubes positioned ina heater previously employed for heating hydrocarbon fluids to atemperature at which carbonaceous material is deposited upon the innerwalls of said tubes which comprises the steps of raising the temperaturewithin said heater to 1250" F. to 1450 F., continuously passing steamthrough said tubes to effect spalling of the carbonaceous materialwithin said tubes, periodically adding controlled quantities of gascontaining free oxygen to said steam to further promote spalling of saidcarbonaceous material, maintaining the temperature conditions and flowof steam until substantially complete removal of the carbonaceousmaterial from said tubes as a granular mass or powder has been effected,reducing the furnace temperature to about 1100 F., continuously passinggas containing free oxygen through said tubes while the heater ismaintained at a temperature of about 1100 F., gradu ally raising theheater temperature to about 1250 F. while maintaining said flow of gascontaining free oxygen through said tubes, thereafter eversing said fiowof gas containing free oxygen and reducing the heater temperature.

4. The method of removing carbonaceous material from tubes positioned ina heater previously used for heating a hydrocarbon fluid to atemperature at which carbonaceous material is deposited upon the innerwalls of said tubes which comprises the steps of subjecting said tubesby means of external heating to a temperature of about G F.,continuously passing steam through said tubes while at a temperature ofabout 1350 F. to eiiect spelling of the carbonaceous deposit within saidtubes, intermittently adding controlled quantities of gas containingfree oxygen to said steam to further promote the spalling of saidcarbonaceous deposit, maintaining the temperature conditions and theflow of steam through said tubes until the spalling action ceases,decreasing the tube temperature to about 1160 F. while continuouslyadding gas containing free oxygen in increasing proportions to saidsteam flowing through said tubes, gradually raising the tube temperatureto about 1250" F. while passing only gas containing free oxygen throughsaid tubes, thereafter reversing said flow of gas containing free oxygenthrough said tubes and lowering the heater temperature while continuingsaid flow of gas containing free oxygen through said tubes.

5. The method of removing carbonaceous deposits from fluid conduits inan oil heater previously used for heating hydrocarbon fluids to crackingtemperatures which comprises the steps of subjecting the conduits byexternal heating to a temperature of about 1350 F., continuously passingsteam through said conduits while at a temperature of 1350" F. to effectspalling of the carbonaceous deposit within said fluid conduits,intermittently adding air to said steam to further promote spelling ofsaid carbonaceous material, maintaining the conduit temperature of about1350 F. while continuing said flow of steam and intermittent additionsof air to eiiect substantially complete removal of said carbonaceousmaterial from said conduits as a granular mass or powder, graduallylowering the heater temperature to about 1lC-0 F. while continuouslyadding air in gradually increasing quantities to said steam, raising thetemperature of the conduits to about 1250 F. while passing only airtherethrough, thereafter reversing said flow of air and maintaining therever ed flow of air while lowering the conduit temperature.

6. The method of reconditioning tubes in an oil heater previously usedfor heating hydrocarbon fluids to conversion temperatures comprising thesteps of gradually raising the temperature within the heater to about1350 F. while passing steam through said tubes, maintaining said heaterat about 1350 F. and continuing the flow of steam to efiectsubstantially complete removal of the carbonaceous deposit from saidtubes, decreasing the heater temperature to about 1100 F. whilecontinuously adding air in gradually increasing amounts to said steam,gradually raising the heater temperature to about 1250 F. while passingonly air through said tubes, reversing the flow of air through saidtubes and maintaining the reversed flow of air while reducing thefurnace temperature.

7. The method of removing carbonaceous material from tubes positioned ina heater previously employed for heating hydrocarbon fiuids to atemperature at which carbonaceous material is deposited upon the innerwalls of said'tubes which comprises the'steps of raising the temperaturewithin said heater to about 1250 F.'-1l50 F., continuously passing steamthrough said tubes to effect spalling of carbonaceous material withinsaid tubes and removing carbonaceous material as a powdered or granularmass from said tubes, maintaining the temperature and flow of steamuntil the spalling action substantially ceases, reducing the tubetemperature to about 1100 F. while decreasing the flow of steam andadding gas containing free oxygen to the steam to cause burning ofcarbonaceous material adhering to said tubes, stopping the flow of steamand passing only gas containing free oxygen through said tubes whilemaintaining the temperature to burn off carbonaceous material adheringto said tubes, gradually raising the temperature of the tubes to about1250 F.-1450 F. while maintaining the flow of gas containing free oxygenthrough said tubes to burn off substantially all of the remainingcarbonaceous material adhering to said tubes not removed by the burningat the lower temperature.

8. The method of removing carbonaceous material from tubes andsimultaneously conditioning said tubes which have been previouslyemployed to heat a hydrocarbon fluid to a temperature at whichcarbonaceous material isdeposited upon the inner walls of the tubeswhich comprises externally heating said tubes to an elevatedtemperature, continuously passing steam through said tubes to effectspalling of the carbonaceous material within said tubes and removal ofcarbonaceous material as a powdered or granular mass, maintaining thetemperature and the flow of steam until the spalling actionsubstantially ceases, lowering the temperature of said tubes Whiledecreasing the flow of steam through said tubes and adding gascontaining free oxygen to the steam flowing through said tubes to effectcontrolled burning of carbonaceous material adhering to said tubes,stopping the flow of steam and increasing the amount of gas containingfree oxygen passing through said tubes while maintaining the tubetemperature at the lowest temperature at which the gas containing freeoxygen will burn off carbonaceous material in said tubes, raising thetemperature of said tubes to a temperature below that temperature atwhich scaling of said tubes occurs and maintaining the flow of gascontaining free oxygen to substantially completely remove anycarbonaceous material adhering to said tubes after the burning off atsaid lower temperature.

9. The method of removing carbonaceous deposits from tubes positioned ina heater and previously used to heat hydrocarbon fluids to a temperatureat which carbonaceous material is deposited upon the inner walls of thetubes which comprises raising the temperature in the heater to about1350 F., continuously passing steam through said tubes while said heateris maintained at a temperature of about 1350 F. to effect spalling ofthe carbonaceous deposit, adding a gas containing free oxygen to thesteam to further promote spalling of the carbonaceous material withinsaid tubes, maintaining the temperature conditions and flow of steamuntil spalling ceases, reducing the heater temperature to about 1100 F.and passing a gas containing free oxygen through said tubes, graduallyraising the heater temperature to about 1250 F. while continuing theflow of gas containing free oxygen and reversing the flow of the gascontaining free oxygen and reducing the heater temperature.

10. The method of removing carbonaceous deposits from tubes positionedin a heater and previously used to heat carbonaceou fluids to atemperature at which carbonaceous material is deposited upon the innerwalls of the tubes which comprises subjecting said tubes to elevatedtemperature while continuously passing steam through said tubes toeffect spalling of the carbonaceous deposits within said tubes andeffect removal of carbonaceous material as a granular or powdered mass,intermittently adding gas containing free oxygen to said steam in aquantity adequate to promote the spalling action but insuflicient inquantity to effect burning of the coke to any substantial extent,maintaining said temperature conditions and steam flow until thespalling action substantially ceases, thereupon reducing the temperatureto which aid tubes are subjected, continuously introducing a gascontaining free oxygen to said tubes while at a lower temperature, andgradually raising the temperature to which the tubes are subjected.while continuing the flow of said gas containing free oxygen to effectcontrolled combustion of carbonaceous material adhering to said tubes.

11. The method of removin carbonaceous deposits from tubes positioned ina heater and previously used to heat carbonaceous fluids to atemperature at which carbonaceous material is deposited upon the innerwalls of the tubes which comprises subjecting said tubes to elevatedtemperature while continuously passing steam through said tubes toeffect spalling of the carbonaceous deposits within said tubes andeffect removal of carbonaceous material as a granular or powdered mass,continuing the application of elevated temperature and continuing theflow'of steam through said tubes until the spalling substantiallycease-s, thereupon reducing the temperature of said tubes and.introducing a gas containing free oxygen thereinto, continuing theintroduction of said gas containing free oxygen while raising thetemperature of said tubes sufficiently to effect controlled combustionof carbonaceous material adhering to said tubes but be- 10W thetemperature applied in the spalling action.

12. The method of removing carbonaceous deposits from tubes positionedin a heater and previously used to heat carbonaceous fluids to atemperature at which carbonaceous material is deposited upon the innerwalls of the tubes which comprises subjecting said tubes to a furnacetemperature within the range of approximately 1250 F. to 1450 F. whilecontinuously passing steam through said tubes to effect spalling of thecarbonaceous deposits within said tubes and effect removal ofcarbonaceous material as a granular or powdered mass, continuing theapplication of said temperature and continuing the flow of steam throughsaid tubes until the spalling action substantially ceases, thereuponreducing the furnace temperature applied to said tubes while holdingsaid temperature approximately within a range of 1000 F. to 1200 F. andpassing a gas containing free oxygen through said tubes and graduallyraising the furnace temperature to about 1250 F. while continuing theflow of gas to effect spallin of the carbonaceous deposits within saidtubes and efi'ect removal of carbonaceous material as a granular orpowdered mass, continuing the application of said temperature andcontinuing the flow of steam through said tubes until the spallingaction substantially 10 ceases, thereupon reducing the furnacetemperature applied to said tubes while holding said temperatureapproximately within a range of 1000" F. to 1200 F. and passing a gascontaining free oxygen through said tubes and gradually raising thefurnace temperature to about 1250 F. while continuing the flow of gascontaining free oxygen to effect controlled combustion of carbonaceousmaterial adhering to said tubes.

ENSLO S. DIXON.

THOMAS E. GARRARD.

HAROLD A. BARR.

